Mechanisms of orientation and graphitization of hard-carbon matrices in carbon/carbon composites
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It now appears generally accepted that the process of stress-induced orientation and graphitization of a thermoset-resin-derived matrix in a carbon-fiber/carbon-matrix (C/C) composite is principally a result of molecular orientation induced during the pyrolysis process as a consequence of the restraint of pyrolysis shrinkage at the fiber/matrix interface by attractive or frictional forces between fiber and matrix. We hypothesize that the critical factor for the formation of lamellar graphite (by subsequent high-temperature heat treatment), instead of fibrillar or isotropic glassy carbon, is a state of multiaxial deformation in the pyrolysis step. Finite-element stress analyses of the relative stresses in the regions of interfilament matrix, as the matrix pyrolyzes from polymer to carbon, reveal patterns of biaxial and triaxial stress consistent with experimental observations of lamellar graphite formation in the matrix by the techniques of optical microscopy, scanning electron microscopy, and transmission electron microscopy. The implications of localized matrix orientation and graphitization for C/C composite properties are discussed in terms of a "duplex" system of composite reinforcement. An example is presented showing crack deflection and blunting at the matrix/matrix-sheath interface produced as a result of such orientation and graphitization.
I. INTRODUCTION The conversion of carbon into graphite is one of the most fundamental aspects of carbon materials science and, appropriately, has been the subject of considerable study.1 Generally speaking, carbonaceous materials can be classified as graphitizing or nongraphitizing. Parallel terminologies include soft or hard, anisotropic or isotropic, and cokes and chars, the last grouping referring only to carbons in their low-temperature states. Except for vapor-deposited carbons, nearly all carbons that graphitize thermally form initially by means of a liquid-crystalline state (mesophase) in which large planar aromatic molecules, produced by polymerization reactions of constituent molecules, stack above each other in a highly ordered fashion to produce the characteristic liquid crystals. The growth in size of the mesophase with continued pyrolysis leads to coalescence of the mesophase and the formation of highly anisotropic coke. Subsequent heat treatment to temperatures above about 2500 °C produces polycrystalline graphite possessing three-dimensional (3D) crystallographic order. Such high-temperature heat treatment is seen to result in refinement of the structural order established in the early mesophase state. A number of asphaltic materials, including pitches, tars, and decant oils as well as many organic compounds, satisfy these conditions for mesophase development. 2798
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 10, Oct 1992
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In contrast, thermosetting polymers, such as phenolformaldehyde, are characterized by random 3D intermolecular crosslinks between polymeric chains. The large number of carbon-carbon bonds that mu
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